Some wireless communications devices, known as back-to-back communications devices, are able to communicate directly with each other without the use of a communications network. Often times, this direct communication is used for short-range ‘push-to-talk’ (PTT) type voice and/or data communications.
Back-to-back communications devices have an option to operate in a direct mode of operation (DMO), where the communications devices communicate directly with each other without using a communications network. This would be contrary to a repeater mode, where the communications devices communicate through a communications network. In repeater mode, a communications device transmits on a receive frequency of a repeater and receives on a transmit frequency of a repeater, while in DMO, a single frequency is used by a back-to-back communications device for both transmitting and receiving. Because of the use of a single frequency for transmitting and receiving, problems (e.g. false carrier detection, false channel detection, and transmission gaps) that occur in both modes are magnified in DMO. Furthermore, problems are magnified when the communications devices are part of a mixed-mode communications system. With an increased variation of modes of operation, the communications devices have a greater difficulty in detecting carriers and channels received.
To illustrate the problems, such as false channel detection and false carrier detection, carrier detection and channel detection need to be understood.
Channel detection is concerned with the detection of attributes that define a channel. In a time division multiple access (TDMA) system, a channel (time slot) is detected when the beginning and the end of the time slot is detected.
Carrier detection is concerned with whether another radio is keyed or dekeyed on a channel, where “key” and “dekey” indicate whether a radio is transmitting (“keyed”) or not transmitting (“dekeyed”). In one conventional method of carrier detection, a device measures received power within a channel and indicates whether the received power exceeds a predetermined threshold. The conventional method of carrier detection is typically designed to detect quickly the presence of energy in a channel, so that the beginning of transmissions will not be missed. The carrier detection is also designed to slowly detect the loss of energy in the channel, so that the receiver is able to tolerate momentary or intermittent fading in the received signal due to continual fluctuations in the strength of a received radio signal. These design characteristics typically cause false carrier detection. However, the carrier detection design characteristics are suitable for a continuous transmission. A continuous transmission may be a FDMA analog, FDMA digital, or a TDMA digital transmission emanating from a repeater as a continuous transmission, as defined in ETSI-DMR, where the repeater transmits a single continuous transmission supporting all channels. This carrier detection strategy is unfortunately not suitable for a series of transmissions that are not continuous. For example, the strategy is not suitable for detecting the presence of a transmission within a time slot of a pulsing TDMA digital transmission.
With reference to FIG. 6, depicted are three cases of TDMA transmissions between which a conventional carrier detector will not be able to distinguish because of its fast-attack and slow-release design characteristics. In Case (a), a transmission sourced by a first communications device is present in channel 1. In Case (b), a transmission sourced by a second communications device is present in channel 2. In Case (c), a transmission is present in both channels (two-channel DMO), where a first communications device is transmitting in channel 1 and a second communications device is transmitting in channel 2. In Case (a) and (b), false detection occurs when using the carrier detector because once energy is detected within a channel, the slow-release characteristic of the carrier detector will not allow the carrier detector to evaluate a second channel independent of a first channel. For example, assume the carrier detector detects a carrier in time slot 1. With a slow-release carrier detector, the carrier detector will indicate that the carrier is present all the way through time slot 2; thus, the carrier detector erroneously indicates a carrier transmission in channels 1 and 2. As is clear from Case (c), the aforementioned scenario is a greater problem in two-channel DMO, because false detection not only indicates the false presence of a carrier in a channel (false carrier detection) it also falsely indicates in which channel the carrier occurs (false channel detection).
A solution for resolving the above-mentioned problems of false carrier detection is to use a different signal qualification technique, such as detection of a synchronization pattern embedded within a signal in addition to the classical carrier detection technique. Furthermore, detection of a synchronization pattern may serve to corroborate an initial conventional carrier detection process. A carrier detector that detects a carrier by additionally detecting a synchronization pattern is hereinafter referred to as a carrier synchronization pattern detector.
It is also desirable to use a synchronization pattern for detection of a channel. A device that enables this method for channel detection is hereinafter referred to as a channel synchronization pattern detector.
Though a carrier synchronization pattern detector is an improvement over the conventional carrier detector, there are still problems with false detection when using synchronization detection, especially in a multiple-channel DMO system. Use of detection of a synchronization pattern to corroborate an initial conventional carrier detection process may decrease false carrier detection for some modes of operation. The technique of combining conventional carrier detection and synchronization detection has been successfully applied to single channel modes and repeater modes. However, when applying such a technique to a multiple-channel DMO, false carrier detection occurs at an unacceptable rate. Similarly, when trying to apply this technique to channel detection, false channel detection occurs at an unacceptable rate. This technique causes false channel detection, because the channel synchronization pattern detector may not be in synchronization with the channel due to a false detection of the channel synchronization pattern. Synchronization detection typically allows for the detection of a synchronization pattern of a channel; however, the synchronization pattern may be detected falsely. Therefore, the boundaries of the channel are determined falsely. Consequently, in a multiple-channel DMO system it is likely that channel detection occurs while spanning multiple channels, resulting in skewed and often unpredictable results as to which channel or channels the transmission is present.
As a result, it would be desirable within a mixed-mode communications system that has back-to-back communications devices that operate in a multiple-channel DMO, to incorporate a carrier and channel detection scheme that provides reliable carrier and channel detection.